The present invention relates to integrated circuit structures and fabrication methods, and especially to the packaging of integrated circuits which use copper metallization.
Background: External Connections for Chip
After an integrated circuit is fabricated, external connections must be formed before the chip is embedded in plastic for protection. With aluminum wiring, which has been the standard for many years, the upper level of wiring for the chip would include bond pads with necessary connections to the underlying circuit. After the protective overcoat (PO) layer is deposited over the chip, holes are etched through the PO to provide access to the bond pads. Depending on the type of packaging used, external connections from the chip can then be made by thin wires which are typically ultrasonically bonded to the bond pad, or by the formation of solder balls which make a direct connection between the bond pad on the chip and the external connector.
However, as device sizes shrink, more and more chip manufacturers are looking to copper to provide the increasingly tiny wiring. One problem which has been encountered is that it is not currently possible to bond directly to the copper bond pad. Copper, unlike aluminum, does not form a self-passivating oxide. When aluminum or gold wires are bonded to the copper, intermetallics are formed which are more resistive and which expand volumetrically, causing cracks and lowering reliability. One solution to this has been to form a barrier layer over the copper bond pad, followed by a layer of aluminum, which can then be bonded to with known methods.
Diffusion Barrier Over Copper Bond Pad
The present application discloses forming a passivation layer over the copper bond pad, using one of several compounds described below. This passivation layer forms a hard, chemically inert layer that prevents the reaction of copper with the metals which are bonded to it without the degradation of the connection which might otherwise occur.